This invention relates to office copying machines and, more particularly, to office copying machines utilizing document scanners in which an improved lens system is provided to enhance the uniformity of intensity of light reflected from the subject.
In some prior art office machines for making copies of documents, a light beam is scanned over a document and the light retro-scattered by the document is gathered by an optical system and directed to a light intensity detector. The detector measures intensity of the light to distinguish the information content in the document and controls the printing mechanism of the copier. Unfortunately, the manufacturing tolerances of the copying machine as well as variations in the position in which a document may be placed allow a document to drift from the point of optimum focus for the optical system. If this is the case, the illuminated portion of the document increases in size whether the document is nearer to or farther from the optical system than is the optimum focus point. The change in distance away from the lens system causes the collecting lens system to subtend different solid angles of collection at different document distances. Consequently, the intensity of light gathered from the same subject varies at different focusing distances.
In addition, in such a machine, the light beam is normally directed upon the document through a central hole in a mirror which also reflects the gathered light to the detector. The hole, in general, removes different amounts of reflected light at different document focusing distances and, consequently, further varies the intensity of the light reflected from the same subject matter at different focusing distances.
The intensity change caused by any of these variations is undesirable because the detection system cannot distinguish those changes from those caused by variations in the information contained in the document.
It is, therefore, an object of the present invention to provide a new and improved lens system for copy machines.
It is another object of the present invention to enhance the uniformity of the light intensity gathered from documents placed at different distances from the optimum focusing point in a document scanner.
It is another object of this invention to provide a light gathering system which subtends approximately the same solid angle of collection at focusing distances which are of practical consequence to a copying machine.